have you read the FAQ?
https://community.bt.com/t5/Home-phone-including-Digital/Digital-Voice-FAQs/td-p/2207485
Bt know what equipment you have/they have sent
existing phone should work
BT will know if they need to send you a compatible hub.
If you are using BT kit they can talk to it, (it’s their kit, after all), and see what it is. If it isn’t suitable, they’ll send you one that is. If you are not using BT kit, well, they can’t talk to it so they know you need a compatible hub.
Any phone will work, (so long as it is not an ancient pulse dial only phone). The phone, or your extension, just plugs into the socket on the back of the Smart Hub 2 instead of the phone socket on the wall.
You only need the adapter if you can’t connect you extension to the Smart Hub 2, for some reason. The adapter plugs into a power socket and functions like a portable phone socket. It uses DECT, (similar to cordless phones), to connect back to the Smart Hub 2.